CHPPWR.THD --- Copyright 1987 by Phil Wheeler An original compilation of Compuserve Model 100 Forum messages for use by Forum members only. An earlier thread (DEDCHP.THD, DL9) discussed the failure of Chipmunk drives due to battery wearout. This is a continuation -- with (eventually!) the designer of the Chipmunk coming in with the "real dope" (Hmmmm!). There is some valuable info here for Munk owners, and some good info on NiCd charging in general. (See DEDCHP.THD for precursor messages). Message range: 158926 to 159673 Dates: 10/22/87 to 11/3/87 Sb: #Dead Chipmnk Fm: Raimon Hennecke 76266,2326 To: Tony Anderson 76703,4062 Tony, if there's really nothing between the nicads and the charging jack, you should never use an 'ordinary power supply' but only regulated chargers. In opposite to your explanations, the charging current is not limited by the resistance of your nicad-battery but only by the circuit designed to charge it. Nicad-batteries are nearly 0 ohms, and so they drain all the current you're offering them. Simple circuits contain only a diode and a resistor, the diode to avoid discharging your batteries backwards through the charger and the resistor to define the charging current. Better ones are transistoroperated. The latter ones would improve the batterie's live because batteries, especially nicads love a more stabalized current. Such a circuit is in the munk's charger, and because of the limited, regulated current it won't work to operate the drive directly. By the way: you should never try to input a higher voltage than 1.5 Volts per cell connected in series except if you want to try to 'zap' an already dead battery to new live - but that's another story! --Raimon-story! --Raimon-- Fm: Tony Anderson 76703,4062 To: Raimon Hennecke 76266,2326 There is really nothing between the external power jack, and the nicads in Chipmunk. And the wall adapter AC supply that was provided with mine is not regulated; it provides a little over six volts loaded, and 11 volts unloaded. I don't think I got into an explanation of limiting charging current, because what I was describing was the difference between slow charge and fast charge nicads. In charging nicads, the charging current increases as a function of the available voltage, and some of the current is disipated as heat. Fast-charge types can be charged at a higher voltage, in order to get current into the cell faster; it is not uncommon to charge at 2.2 volts per cell in 5-hour chargers, for example; 2.7 (3 hour) or even 3.0 (1 hour). One should consult the spec sheets for the particular cell involved, of course, or use only a charger supplied by, or recommended by, the product manufacturer. In all cases I know about, the fast-charge types cannot be recycled as many times as slow-charge types. I have difficulty accepting the need for a regulated current charger, since nicads prefer to "taper off" as they come up to full charge. It's sometimes referred to as "automatically going into trickle charge mode". As the cell voltage approaches the charging voltage, it draws less current, thus the charger only provides enough current to maintain full charge, preventing self-discharge. I'm sure you meant regulated _voltage_, which I wouldn't argue with. Of course, we may be talking about two different approaches here, and which one is right depends on what you want. The original message in this thread indicated you shouldn't use a power supply with more current capacity than the one provided as original equipment, because it would "force" a higher current into the batteries. Technically incorrect. Fm: Raimon Hennecke 76266,2326 To: Tony Anderson 76703,4062 Tony, to clear things up and to avoid dangerous misunderstanding, I'll try to go into more detail now. The specs for a Panasonic NC-Cell for example say that it has a resistance of 5 milliohms, that's 0.005 Ohm. Now, assuming you have 5 cells in series to get 6 Volts, the resistance of this 'NC-pack' would be 0.02 Ohm. If wired like you explained, that is to connect any power source to charge them as long as it has the right voltage (6 Volts) but the current (not the Voltage!) not regulated or limited, the nicads would draw (according to Ohm's law) 6V:0.02ohm=300 Amperes - or in other words, all the current which is available from the supply as long as it provides less than 300 Amperes. So you have to limit this current by at least a resistor. In our example, to charge a 6V-500mAh-pack with 50mA (for a 15 hrs standard charge) you must use a resistor of 6V/0.05A=120 Ohms to limit the current supplied by the charger. You can forget the additional 5 milliohms of the batteries to calculate this. And, of course there is no need of a regulated Voltage, as long as it doesn't go above 1.5 Volts per 1.2V-cell when loaded. Obviously all this applies to the Munk's charger, and because it also doesn't operate the drive itself, the limitation of current also seems to work o.k. I recall an incident which happened to me over 15 years ago, when the AC-adapter for my little TI-calculator failed. I didn't realize that it was only a charger and connected another supply to the calculator, which damaged both, the power supply as well as the nicad-battery pack (which was very expensive that days) You should never mix up 'chargers' with 'ordinary power supplies' because they work differently! The Munk's charger isn't simply a 'weak power supply' but an especially for charging purposes designed unit. I am using NiCads very much over years now - so if you should have any further questions I would love to answer them. All the best from Germany: --Raimon-best from Germany: --Raimon-- Fm: Tony Anderson 76703,4062 To: Raimon Hennecke 76266,2326 Rather than cut my power supply adapter in half, I'll call Glen at Holmes on Monday, and find out exactly what the little beast does, and how it's configured. My guess, however, is that it's an unregulated supply. I currently have two or three dozen nicads in various devices around here, and recharge them often; many with a regulated bench power supply, with no ill effects. I also have a handy-talkie that I use at work, that is fast charged daily (5 hours), in a factory-supplied charger... ten cells for a 12 volt pack, and it's charged with 22 volts. Fm: Raimon Hennecke 76266,2326 To: Tony Anderson 76703,4062 Tony, let me know what Glen will say. In respect to your experiences with other devices: of course the circuit to cut the current down is often built in there and not in the charger to allow operation (with or without the cells ) and charging with the same adapter. On the other hand: I know that NiCads are often charged with too high voltages. Or with too much current. But to determine the correct charging time, you have to know how much current is used. You can overcharge NiCads very easily - and that happens very often which results in a life of 2 years or less, and the user gets disappointed wondering how this could happen. Also the often heard story of the 'nicad's discharge memory' is nothing else than a result of permanent overcharge. For example: if you give batteries which are only used up half, a full standard charge, you overcharge them by 50%, which, if done over and over again, results in a loss of capacity sometime. The only way to charge them continuously without harm is to do this with only 1/30 of the nominal capacity, that is a current of 16-17 mA for an 500mAh-battery. The best way of course would be an automatic charger which detects the voltage of the cells and when reaching the peak of 1.45V, finishes or goes down to the 1/30. But those chargers are much more expensive and so unfortunately you don't find them very often. Of course - as everywhere in life you'll also find here the big difference between theory and practice. Nobody reading this should now kick himself finding out that he may have overcharged his accu's. They are not that sensitive! But I think one should be aware of the theoretic correlations to understand more about capabilities and limitations of these little useful accu-thingies. --Raimon---Raimon-- Fm: Tony Anderson 76703,4062 To: Raimon Hennecke 76266,2326 I called Glen, and he advises us that the AC adapter/battery charger for the Chipmunk is UNregulated; it is primarily a trickle charger for the nicads; and that they can be left on trickle charge indefinately. Also, should you wish to convert the chipmunk for AC-Only operation, the way to do it is to unplug the nicad stack, so it is no longer in use, and supply power to the Chipmunk with a 6 to 7.5 volt supply capable of providing 1 amp. It is not necessary that the supply be regulated, as there is an internal regulator in the disk drive power chain that drops the supply voltage to 5 volts anyway. Fm: Larry Holmes 72276,1666 To: Tony Anderson 76703,4062 You are right, it is an unregulated supply. At the time the 'munk was designed the battery suppliers informed us that it was OK to trickle charge the batteries using an unregulated charger. I now believe that it is "OK", but not the preferred method. For various complicated reasons, a more sophisticated charger would be better, but of course, it would also cost more. If you look at a enith (Zenith) laptop, for example, you will find that it has a temperature sensor wrapped in with the batteries. The batteries are charged with a heavy current until they reach full charge, then a very slight trickle current maintains them if the power is left connected. The temperature sensor allows the high charge rate because it adjusts the charging current downward if the batteries get too hot. The Chipmunk has a simple supply but it was selected for its equivelant d.c. resistance and dc voltage output at light loads. The dc resistance of the secondary of the transformer and the diodes automatically limits the charging current into the ni-cads. When the batteries charge up, their terminal voltage rises, and the voltage difference between the charger and the batteries drops, reducing the charging current. Eventually equilibrium is reached when the battery terminal voltage stops rising; this is the point at which this simple method falls short. The chipmunk adapter continues to try to charge the batteries, and they heat up. The adapter is chosen to limit this effect to below the danger point, and the battery manufacturer insists it is OK to do this. But there is no doubt that a more sophisticated charger would both prolong battery life AND keep the charge cycle at maximum for the life of the batteries. Such a charger would cost four times as much as the 'munk ac adapter, so perhaps it is better to replace the batteris every coupld of years????? Anyway, that is the rationale behind the charger for the chipmunk. It is compromise, to be sure, and it works fine, but there is no doubt it could be improved......Larry Fm: Larry Holmes 72276,1666 To: Raimon Hennecke 76266,2326 Alas, Tony is right, the charger for the Chipmunk is just a simple unregulated power supply. This charging technique is approved by the battery manufacturer and it works OK, but, a more sophisticated circuit certainly would improve battery life and discharge time. Cost constraints kept us from putting the more sophisticated circuit in; it would have raised the price of the drive enough to bother a lot of people, and the price was already high enough! Larry Holmes, CHipmunk designer. Fm: Larry Holmes 72276,1666 To: Tony Anderson 76703,4062 Ooops....it is not advisable to leavve the charger on indefinitely. It will shorten the discharge time for the batteries. Put it on, charge 'em up, then take it off and run them down if possible. Of course, if you rarely use the batteries, it doesn't matter...... I would advise a dc power supply of 7 to 9 volts; you could go as high as 12 (but no more!) as the switching regulator in the chipmunk will convert any voltage from 4 to 12 volts to the levels it needs. If you put 6 volts in it will work but you will be running at the bottom end of its range, it will get hotter and work harder. Better to put in ahigher voltage and let things run cooler (conversion losses stepping up to 12 volts cause most of the heat and strain). Fm: Doug Pratt (ModelNet) 76703,3041 To: Larry Holmes 72276,1666 Larry, the trickle charge method you describe for the Munk batteries works fine, in my experience. I've charged 6 cell 1200 mAh packs for weeks at the same rate and never noticed a detectable increase in heat. I have a really neat little peak detector charger that supplies a pulsed 3-amp charge current until it detects the knee of the charge voltage curve, then shuts down to trickle. Costs $19.95 retail. Might be a nice accessory for the Munk! Fm: Tony Anderson 76703,4062 To: Larry Holmes 72276,1666 I have no problem with the way the charger works in the Chipmunk. It's the way most "chargers" have worked for most nicad-served devices for years. It's common among both consumer devices and technician-built devices. The fact that the nicads eventually fail and need to be replaced is no problem; nobody expects them to last forever. ... at least nobody SHOULD! A three year life of the nicad stack in the Chipmunk is par for the course, as far as I'm concerned, and it seems to be the case with the few reports we've had of nicad failure. As you no doubt know, there are technical tricks that can (possibly) revive certain nicads, in certain cases. And many technicians will routinely replace individual defective cells, but I hesitate to even get into that discussion. Looks like I will be going over to a 9 volt AC-only supply, since I NEVER use the Chipmunk as a portable anymore. Fm: Eiji Miura 76703,4311 To: Doug Pratt (ModelNet) 76703,3041 I used to have a quick nicad charger which charged Tamiya's nicad pack (for RC cars) in about five minutes from a cigarette lighter in a car. Can something like that used to recharge Chipmunk nicad? I want something that can recharge Chipmunk battery faster. Fm: Doug Pratt (ModelNet) 76703,3041 To: Eiji Miura 76703,4311 Well, I would be hesitant to use that specific charger. It's designed for 6-cell 1200 mAh packs, and the Munk pack has 5 cells. 1200 mAh cells are pretty tough, but I like to baby them anyway. What I had in mind are some of the hi-tech chargers coming out for RC cars and electric powered airplanes. Just saw an ad for a new one with a built-in voltmeter. That's the ideal way to do it. There are several units with heat sensors, which terminate charge when the pack hits 120 degrees, but you'd have to open the Munk to charge. I thik the best thing to do is put in a medium-fast charge current, like 2 amps, which should give you full charge in 4-5 hours, and monitor the voltage. If the voltage rises or holds steady, you're still charging. When the voltage drops, you're fully charged, and the current is begin converted into heat. Fm: Raimon Hennecke 76266,2326 To: Larry Holmes 72276,1666 Larry, thank you for your information which clarifies some things - it obviously explains the reasons of the reported limited lifetime of some munk's nicads. But as I already explained in an earlier message of this thread, you won't find a more 'sophisticated charging circuit' very often. On the other hand I can't always understand the reasons for omitting inexpensive but very important parts in several products as in the new Atari ST-line, where, for example, they left a simple IC out which could cause a damage of the sound ship if you connect a printer while leaving the computer on. Of course this message doesn't mean I don't like your products. I am very satisfied with them since I got your very useful 48k ram-expansion board several years ago for my trusty Model 1. Too bad that you've left your company as I heard recently - I wish you all the best in the future. Greetings from Germany: --Raimon-from Germany: --Raimon--